In the high-frequency radio communication technology, a circuit called a filter is used to separate various signals into necessary signals and unnecessary signals by allowing signals at predetermined frequencies to pass through the filter and blocking other signals. In general, a filter has a fixed central frequency and a fixed bandwidth as design parameters.
Filters are used in various types of radio communication devices. In order for a radio communication device to function at various frequencies and with various bandwidths, a filtering function has to be provided at various central frequencies and with various bandwidths. A possible method to achieve this is to use a switch to switch among a plurality of filters having different combinations of central frequency and bandwidth that are conventionally mounted in the device. According to this method, the same number of filters are needed as the number of the combinations of central frequency and bandwidth, so that the circuit size increases. As a result, the device becomes large. In addition, according to this method, the radio communication device cannot serve the function under conditions other than the combinations of central frequency and bandwidth of the conventionally mounted filters.
To solve the problems, according to a technique disclosed in Japanese Patent Application Laid-Open No. 2004-7352 (referred to as Patent literature 1 hereinafter), a piezoelectric element is used in a resonator in the filter, and a bias voltage is externally applied to the piezoelectric element to change the frequency characteristics (resonance frequency) of the piezoelectric element, thereby changing the bandwidth of the filter.
According to a technique disclosed in T. Scott Martin, Fuchen Wang and Kai Chang, “ELECTRONICALLY TUNABLE AND SWITCHABLE FILTERS USING MICROSTRIP RING RESONATOR CIRCUITS”, IEEE MTT-S Digest, 1988, pp. 803-806 (referred to as Non-patent literature 1 hereinafter), a resonator is used that comprises two microstrip lines arranged in a ring with the ends of one opposed to and connected by a PIN diode to the ends of the other to provide a filter capable of changing the central frequency.
The variable filter disclosed in Patent literature 1 has a bandwidth of a ladder filter. However, the extent to which the central frequency of the filter can be changed is as small as about 1% to 2% because of the limitation by the characteristics of the piezoelectric element, and thus, the bandwidth can be changed only to similar extent and cannot be substantially changed.
The filter disclosed in Non-patent literature 1 can change the central frequency but cannot substantially change the bandwidth.
In addition, according to the prior art, circuit elements (such as a resonator and a filter) in a circuit have their respective fixed roles, and it is difficult to make the whole or some of the circuit elements function as components (such as a transmission line) other than themselves.